1. Field of the Invention
The present invention relates to an image sensing apparatus and an image sensing apparatus control method configured to carry out firing of a photometric pre-flash or the like, and more particularly, to an image sensing apparatus and an image sensing apparatus control method adapted to the use of an image sensing unit in which a plurality of photoelectric conversion devices like CMOS sensors is arrayed in a two-dimensional layout and that is capable of shifting an exposure period in units of one line or a plurality of lines.
2. Description of the Related Art
In recent years, image sensing units configured by integrating image sensing devices in high densities has been used in video cameras, digital cameras and the like, and it has become possible for a plurality of frames of image data to be recorded on a recording medium, and for image data to be printed out, displayed, and so forth.
An AE (Automatic Exposure) function is employed in these types of video cameras, digital cameras and the like, so as to expose during daylight at an optimum exposure condition obtained by measuring the light quantity of the imaging area.
In addition, such cameras are equipped with auxiliary lighting devices such as a flash or an electronic strobe and under low-light conditions, such as at night or indoors, flashlight is emitted when the image sensed (flash photography).
In reality, it is difficult to control the exposure instantaneously using the AE function when firing the flash. Therefore, a pre-flash firing is carried out, the quantity of light at that moment is measured, and automatic exposure control completed prior to the firing of the flash, after which the actual photography is carried out. Typically, since the quantity of light of the flash cannot be measured accurately if the pre-flash is fired outside the charging time of the image sensing unit. Thus the pre-flash firing is synchronized with the timing of the firing of the flash and the image sensing unit is charged.
In recent years, in video cameras, digital cameras and the like, in order to obtain high-resolution images, reduction in the size of the image sensing device cell using miniaturization processes is being energetically pursued.
At the same time, because the photoelectric conversion signal output per cell decreases as the cell size decreases, an amplifier-type image sensing unit in which the photoelectric conversion signal can be amplified has attracted attention. This type of amplifier-type image sensing unit includes such two-dimensional image sensing units as BASIS, MOS, SIT, AMI, CMD and other such XY address-type sensors.
In addition, as other two-dimensional image sensing units, CCD (Charge Coupled Device) sensors are widely used due to their characteristics: easy to integrate in high density and high S/N ratio.
In digital cameras and the like equipped with a two-dimensional image sensing unit, when an image is sensed, the device adjusts an exposure condition appropriately for that image in order to set the exposure period according to the sensitivity of the image sensing unit. When setting the exposure period, the device measures the quantity of light striking a portion of the image sensing devices arranged in a two-dimensional layout and changes such image sensing parameters as the aperture or the like until the measured value reaches a target value, thus finding the optimum values of the image sensing parameters. This operation is the AE (Automatic Exposure) operation.
When performing AE with a digital camera that uses a CCD sensor, all pixel data is read out and stored on a storage medium, from which predetermined block areas are extracted, and, after charging once, is compared to an appropriate, predetermined exposure level. Then, the image sensing parameters, such as the shutter speed, aperture, and so forth, are changed and the read-out pixel data is once again subjected to block area extraction and compared to a predetermined exposure level. By repeating this operation multiple times, the condition at which the block area exposure level becomes the predetermined exposure level is set as the optimum exposure condition.
A description is now given of an AE operation according to an image sensing unit that uses a CCD sensor, with reference to FIGS. 24A-24C. The sensor read method may be either interline transfer or full frame transfer. An image signal read out in time sequence from a CCD sensor 1601 is converted into a digital image signal by an A/D converter 1602 and a single frame is written to a frame memory 1603.
When performing AE using center-weighted metering, in particular only the central block area is read, and an integrator 1604 then integrates image signal levels in that block area to obtain a total integrated value for that block area. A determination circuit 1605 then compares the total integrated value of the block area and a preset, predetermined exposure level, and if the results of that comparison show that there is a difference between the two values, that difference is then output to an exposure condition setting circuit 1606. From among such exposure conditions as the exposure time/shutter speed the aperture value, and a charge time of the CCD sensor 1601, the exposure condition setting circuit 1606 changes the shutter speed or the like, for example. Under exposure conditions determined by the settings of the exposure condition setting circuit 1606, of the image data obtained once again from the CCD sensor 1601, the exposure integration value of the block area is detected, compared and determined by the determination circuit 1605, and the process repeated until the difference falls below a predetermined value, thereby obtaining the optimum exposure condition.
This AE operation is shown in the timing chart shown in FIG. 24B. In FIG. 24B, (1) shows the exposure time (electrical charge accumulation period) at low level period. At AE evaluation time (2), a period in which a block area is read from the frame memory 1603 and integrated after electrical charge accumulation and A/D conversion, is shown at high level. At the AE evaluation time AE values (3), the dotted line indicates the predetermined AE value, which, if graphed, would appear as shown in FIG. 24C, where, at a third exposure detection value, the AE value is too high, whereas the exposure conditions at a fourth AE value are made the AE conditions, thus enabling execution of optimum AE.
Nevertheless, in recent years, the number of instances in which CMOS sensors are used for the two-dimensional image sensing unit has been increasing because they are inexpensive and do not require complicated timing generation circuits, and moreover because they can operate a single power source and consume little electric power and the like.
However, because CMOS sensors read signals in units of lines, starting time of the photoelectric charge accumulation will be different for each line. As a result, accumulation timing for AE shifts for each line and accumulation period of a block area for AE evaluation and the pre-flash timing may get out of synch. In other words, of the lines contained in the block area for the purpose of AE evaluation, there appear lines that are not accumulated during the pre-flash.
As a countermeasure thereto, for example in Japanese Patent Application Laid-Open No. 2000-196951 there is disclosed controlling so as to fire the pre-flash for all block areas by using the pre-flash in periods in which the lines read from the block area overlap in time.
However, where the shutter speed is fast, such as during daylight synchro photography, there are no periods in which the lines read from the block area overlap in time. In such cases, when the pre-flash is fired arbitrarily, both the photoelectric charge from photographic light composed of outside or environment light and reflected light of the flash light and the photoelectric charge from photographic light composed only of outside light are to be accumulated at the CMOS sensor, making accurate detection of the quantity of light impossible as a result.